A display of historical Chinese astronomical instruments on the roof of the Beijing Ancient Observatory in Beijing, China, an astronomical observatory that dates from the 1400s. From left to right: azimuth theodolite (1715) to determine azimuth and altitude (elevation angle) of celestial objects; altazimuth (1673) to determine the azimuth of celestial objects; ecliptic armilla (1673) to measure ecliptic longitude difference and latitude of celestial objects.